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512 WIRING FOR ALTERNATING CURRENT [Cn. XIII
moist, many small bubbles will appear around the negative pole. But the greenish color given at the positive pole is the most certain. Turn off the current and mark the positive wire red.
With the other chemical tests (A, B, C) the indications are in no way dependent on the metal forming the wire, but with the potato test the poles entering the potato must be copper or contain copper.
§ 704. Wiring the three-wire automatic lamp of the Bausch & Lomb Optical Company. — This lamp is regulated wholly by electricity, there being no clock-work. In wiring the lamp one proceeds exactly as described above (§ 693-700), except that a wire is carried from the positive side of the switch to the middle binding post of the lamp directly. Another wire from the same point is carried down to the resistor or rheostat, and from the rheostat a wire to the positive or upper binding post of the lamp. From the negative pole of the switch a wire is carried directly to the lower or negative binding post of the lamp. This wiring gives the full voltage of the line for the electric mechanism governing the lamp (see fig. 145).
WIRING FOR ALTERNATING CURRENT
§ 705. This is precisely as for direct current, and one does not have any trouble about the polarity. It makes no difference which supply wire is connected with the upper carbon and which with the lower.
§ 706. Insertion of the rheostat, inductor or other balancing device. — It makes no difference in which of the lead wires the rheostat, etc., are inserted. Just as with direct current, however, the balancing device must be inserted along one wire (fig. 281), otherwise the current would not traverse the entire circuit.
§ 707. Position of the rheostat, etc. — The balancing effect of the rheostat is the same no matter where it is installed in the special circuit for the arc lamp. For convenience it is frequently put on or near the projection table. This is especially necessary if the rheostat is adjustable. With a fixed rheostat it is sometimes safer